PSA processes have been employed for many years to separate the components of a gas mixture. PSA processes are carried out in an elongate vessel which has a feed gas inlet end and a nonadsorbed gas outlet end and which is packed with an adsorbent which preferentially adsorbs one or more of the components of the gas mixture. The gas mixture is passed cocurrently (from feed gas inlet to nonadsorbed gas outlet) through the vessel, thereby removing the preferentially adsorbed component from the gas stream. A product gas enriched in the component or components that are not preferentially adsorbed passes through the adsorbent bed and exits the bed through the nonadsorbed gas outlet. The adsorbed component initially accumulates at the inlet end of the bed, and as the adsorption step proceeds, the adsorbed component forms a front which gradually moves toward the nonadsorbed outlet end of the bed. When the adsorbed gas front reaches a certain point in the bed the adsorption step is terminated and the adsorbent is regenerated by desorbing the adsorbed component from the bed. This is generally accomplished by countercurrently depressurizing the bed, and/or by countercurrently purging the bed with nonadsorbed component gas. When the bed is regenerated to the desired extent, the cycle is repeated.
A typical PSA cycle includes a pressurization step, in which the pressure in the adsorption vessel is raised to the pressure at which it is desired to conduct the adsorption step of the process, by introducing a gas (usually the gas mixture being separated) into the vessel; an adsorption or production step; and a bed regeneration step. The cycle may include other steps, such as multiple pressurization and depressurization steps.
In conventional PSA processes, the adsorption step is generally conducted at moderate to high pressures, e.g. pressures in the range of about 5 to about 20 bar, absolute(bara), and the bed regeneration step is frequently carried out at or below atmospheric pressure. These processes are generally efficient and result in the production of consistently high purity nonadsorbed gas product. Such processes are, however, energy intensive, since considerable energy must be expended to compress the feed gas to the adsorption step operating pressure.
Low pressure PSA processes with low adsorption pressure to regeneration pressure ratios have recently been developed. These processes are generally operated with adsorption pressures up to one to three bara and bed regeneration pressures of about atmospheric pressure. The feed gas can be easily pressurized to these pressures by means of low energy equipment, such as blowers, and since the bed is regenerated at atmospheric pressure there is no need to use high energy vacuum generating equipment. In such low pressure processes, it is common to use a portion of the nonadsorbed gas product produced in each cycle to purge the bed of adsorbed gas component in order to enhance process performance.
It is highly desirable that the variation of quality of product gas produced in the various beds of a multiple bed adsorption system processes be very low. However, when multiple bed adsorption systems are used for adsorption processes that are carried out at low adsorption pressure to vent pressure ratio operating conditions, e.g. 3 bara/atmospheric pressure, significant variations in product quality are experienced. This does not present a problem when the equipment is used for moderate or high pressure adsorption processes, since the variation of product quality diminishes with increasing adsorption pressures.
U.S. Pat. No. 4,472,177 discloses a vacuum swing adsorption process for producing oxygen and nitrogen from an air stream. According to the disclosure of this patent, nitrogen is adsorbed from air which is at near ambient pressure to produce oxygen as nonadsorbed product. After completion of the adsorption step of the process nitrogen is passed through the beds to rinse oxygen from the void spaces in the beds. The rinse step is terminated when low oxygen is detected in the purge gas effluent.
Copending U.S. patent application Ser. No. 08/189,008, filed Jan. 28, 1994, now U.S. Pat. No. 5,490,871, discloses a process for preventing excessive loss of purge gas in a PSA process by analyzing the purged waste gas stream from the process for purge gas, and terminating the purge step when the concentration of purge gas in the waste stream reaches a preselected volume of the waste gas stream.
Because of the attractiveness of low pressure adsorption processes, improvements that reduce product quality variation in multiple vessel systems are constantly sought. This invention presents an efficient and cost effective method of accomplishing this goal.